With rapid development of mobile communications, a scheme in which multiple systems share a common station as well as a common antenna feeder resource has gotten its population by more and more operators. By this way, advantage of sharing resource and reducing system device cost can be obtained. Within a system in which 2G/3G sharing a common antenna feeder, dual frequency multiplexer is a necessary microwave component that mainly serves to multiplex/de-multiplex signals of different systems so as to save the length of feed cable, simplify system construction and reduce cost. In addition, power is supplied to the devices at base station tower via a radio frequency cable and accordingly, the multiplexer connected with the feed cable must have the ability of passing direct current there through.
With reference to the schematic diagram of FIG. 1, a multiplexer is a microwave component having three ports, i.e., two direct current feed circuits and two RF signal circuits, wherein each direct current feed circuit is constructed by a lump parameter low pass filter, a switch and a lightning protection device. The low pass filter is used to suppress RF signal at high frequency such that control signal at certain frequency (such as 3 MHz) can pass the filter with ease. The switch serves to selectively conduct direct current there through. The RF signal circuit consists of a blocking capacitor and a band pass filter. The band pass filters of respective two RF signal circuits have their band pass range be set so as to be suited to frequency range of two signals to be multiplexed. During operation, the signal input from a common port Port1 is shunted to Port2 or Port3 according to the frequency range. Alternatively, the signal input from ports Port2 or Port3 may also be combined and output via the port Port1.
The RF signal frequency range of the system in which 2G/30 sharing a common antenna feeder is 806 MHz-960 MHz and 1710 MHz-2170 MHz respectively. By now, to obtain such wide a work frequency band and capability of passing direct current, most multiplex products employ dielectric substrates and realize it by micro-strip circuits. The disadvantages of product of this type include large bulkage and low power capacity. Moreover, inactive inter-modulation is greatly depended on property of dielectric substrate material and therefore, it is difficult to control the dielectric substrate material during batch production.